1. Field of the Invention
The present invention relates to a zoom lens, a lens barrel and an image pickup apparatus, and more particularly to a so-called zoom lens as an interchangeable lens of a digital single-lens reflex camera, a lens barrel including the zoom lens, and an image pickup apparatus including the lens barrel.
2. Description of Related Art
For example, a so-called normal zoom lens of a digital single-lens reflex camera is roughly classified into three types of ones, that is, a high-grade type of one, a high-magnification type of one, and a popular type of one. In this case, in the high-grade type of normal zoom lens, in a zoom entire area, brightness is F2.8, and a zoom factor is about 3×. In the high-magnification type of normal zoom lens, brightness in a wide-angle end is about F3.5, and a zoom factor exceeds 10×. Also, in the popular type of normal zoom lens, brightness in a wide-angle end is in the range of F2.8 to F4 and brightness in a telephoto end is in a range of F4 to F5.6, and a zoom factor is in the range of about 4× to about 5×. 
In recent years, of the three types of normal zoom lenses, in particular, a penetration rate of the high-magnification type of normal zoom lens has increased. However, a demand for a normal zoom lens which is differentiated from the high-magnification type of normal zoom lens and which has both the high-grade type-like brightness and the popular type-like zoom factor of about 4× to about 5× has increased.
Now, with regard to a construction of a refracting power arrangement of the normal zoom lens, in the case of the normal zoom lens having the brightness of F2.8 in the zoom entire area, a negative lens group preceding type, and a positive lens group preceding type exist. In the negative lens group preceding type, a first lens group is a negative lens group. The negative lens group preceding type zoom lens, for example, is disclosed in Japanese Patent Laid-Open No. 2000-221399. Also, in the positive lens group preceding type, a first lens group is a positive lens group. The positive lens group preceding type is disclosed in Japanese Patent Laid-Open Nos. 2005-181556 and 2007-264381. In each of the high-magnification type of normal zoom lens and the popular type zoom lens, a zoom lens having a four group construction in which a positive lens group, a negative lens group, a positive  lens group, and a positive lens group are arranged in order from an object side has become a mainstream.
In addition, any of the negative lens group preceding type of zoom lens, and the positive lens group preceding type of zoom lens is constructed in such a way that a stop moves along with zooming, and thus the stop is located on the object side when the zooming is carried out from a wide-angle end to a telephoto end.
Therefore, when the brightness is desired to be set as being constant, for example, set at F2.8 in the zoom entire area, control needs to be carried out in such a way that an opening size of the stop becomes larger on the telephoto side because a position of an exit pupil moves away from an opening area in a phase of the zooming from the wide-angle end to the telephoto.